Code conversion



Sept. 30, 1953 H. M. STRAUBE 2,854,657

CODE CONVERSION Filed April 13, 1954 2 Sheets-Sheet 1 l g I E U 27L/M/TER T PROTECTIVE FILM mo/v WIRE (CAT/100E) (ANODE) 0/0 (ELECTROLVTE)lNVENTOR H. M. 5 TIM UBE NW may ATTORNEY P 30, 1958 H. M. STRAUBE2,854,657

CODE CONVERSION Filed April 13, 1954 2 Sheets-Sheet 2l5/k/3//l/098765432/ CLIPPER 27 c0. /5 I L/M/ TE R F/G 4 a0 3/ as as I AC ODE R DE MIXER 4 .22 as :4 i? WE W n I TIMING GEN. v

I H J Q m V l l I I f I lNl/ENTOR H. M. STRAUBE' Br A TTORN EV UnitedStates Patent CODE CONVERSION Harold M. Straube, Mendham, N. J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application April 13, 1954, Serial No. 422,835

11 Claims. (Cl. 340347) This invention relates to code conversion andespecially to the conversion of a time code to a space code. Its generalobject is to effect such code conversion at such a leisurely rate as toimpose no special high speed requirements on the associated registeringapparatus. A related object is to eflect such code conversion, and togenerate unequivocal space code registration, without resort todifferential amplification or attenuation of intermediate signals whichappear in the apparatus.

By time code a sequence of pulses which follow one another in time, e.g., on a common conductor, is intended. By space code there is intendeda distribution of energization conditions which may be simultaneous, e.g., among a plurality of difierent conductors. The codes may be in thesame language in Which case the conversion is analogous to the action ofa stenotypist, who hears with her ears the words of an English sentenceas they are spoken one by one, and types the printed text equivalent.But the conversion may also include a translation from one code languageto another, e. g., from the binary language to the decimal language, orvice versa. This is analogous to the action of a bilingual stenotypistwho may hear with her ears a sentence spoken in German and type itsprinted text equivalent in English, translating from German to Englishas she goes.

.An electromagnetic transmission line provided with lateral taps may beemployed for time-to-space code conversion. For each of a sequence oftime-coded input pulses a wave may be caused to travel from one end ofthe line to the other giving rise to an output pulse on each lateral tapas it passes by. The distribution of the various simultaneous outputpulses as between the several lateral taps constitutes the space code.Translation from one code language to another may be accomplished byinterconnections among the taps inaccordance with a prescribed codetranslation pattern.

Electromagnetic transmission lines are characterized by very highvelocities of propagation. In circumstances where delays of the order ofa microsecond or less are required, this is of great advantage. But insome circumstances the delays required lie in the range from second to asecond or more. To achieve such delays with a conventionalelectromagnetic transmission line would necessitate a line ofenormouslength. Such a line is out of the question as a practicalmatter, both because of the excessive space requirements which itimposes and because any wave is greatly attenuated in the course of itspropagation from one end of such a long line to the other end, with theresult that the pulses derived at the successive output taps aresuccessively diminished in amplitude, necessitating a large amount ofdifferential amplification.

What is needed in such a situation is some extended element along whicha disturbance of suitable character may be propagated slowly and withoutattenuation, and a particular object of the present invention is toprovide one. The invention is based on the realization that, with properattention to design features, an electrochemical Patented Sept. 30, 1958element may be endowed with these properties. In particular, theinterface between a solid of one chemical constitution such as a metaland liquid electrolyte of another constitution, such as an acid ofsuitable proportions, constitutes an extended medium which isquasistable in the sense that, upon immersion of the metal in the acid,a protective film at first forms on the metal which, however, may bebroken down by administering an electrical pulse to it. The breakdownproceeds to travel along the interface away from the point at which itwas initiated. Thus, if the breakdown is initiated at one end of a rodor wire, the breakdown is propagated as an electrochemical disturbancetoward and ultimately to the other end. As the disturbance proceeds, theprotective film is continually reformed immediately behind thedisturbance. A good example of such a device comprises a rod or wire ofiron immersed in a bath of nitric acid of moderate concentration, thewire and the acid being together supported in a vertical position by aglass tube. The propagation speed of an electrochemical disturbanceinitiated at the lower end of the wire is of the order of A to 10 metersper second, its exact value being determined by the concentration of theacid, the outside diameter of the wire, the inside diameter of thesupporting vessel, the temperature of the apparatus and the thickness ofthe protective film, which in turn depends in part on the time which mayhave elapsed since the last disturbance. As the disturbance passes eachof a succession of taps which may be inserted through the wall of theglass tube, it gives rise to an output pulse on that tap. It is afeature of the invention that the amplitude of an output pulse derivedat any tap depends only on conditions in the neighborhood of that tapand is to a large extent an electrochemical constant. It is entirelyunrelated to the distance which separates the tap from the point oforigin of the disturbance, i. e., the input end of the device.Consequently, with like constructions for the several taps, theamplitudes of all the output pulses are identical, and with properselection and spacing of components may be caused to follow one anotherat intervals of 1 second or so with compact apparatus.

The comparatively long recovery time required by the electrochemicaltransmission line serves to ensure against the propagation of a spuriouswave originating in an unintended initiating pulse due to noise or animage of a desired pulse. On the other hand, it introduces a delaybetween successive operations of the apparatus which might beobjectionable in a system of the prior art. To obviate such objections,the invention provides a novel approach to the code conversion operationin that only a marker pulse is propagated over the transmission line, togive outputs at the several taps and, by way of a codeconversion matrix,at the several output points of the apparatus. Such outputs are thusalways the same, independent of the pulse group to be converted. Thepulse group itself is then balanced against such output pointindications as to give an unequivocal indication of that one of thegroup of output points at which the balance is complete.

The invention will be fully apprehended from the following detaileddescription of a preferred illustrative embodiment thereof, taken inconnection with the appended schematic diagrams in which:

Fig. 1 shows a time-to-space code converter including, as one of itselements, an electrochemical transmis sicn line, but omitting thedetails of a matrix which interconnects the lateral taps of the linewith the output points of the apparatus;

Fig. 2 explains the mechanism by which an electrochemical wave advances;

Fig. 3 shows the same apparatus as Fig. 1 and pro- 3 vides the detailsof the matrix omitted from Fig. 1 but omits others; and

Fig. 4 is a block schematic diagram showing a combination of apparatuscomponents which may be employed to insert a marker pulse of aparticular form between successive time code pulse groups to beconverted.

Referring now to the drawings, Fig. 1 shows a glass vessel or test tube1 containing a liquid solution 2 of nitric acid in which a rod or wire 3of iron is immersed. The rod 3 may be centrally supported within thetube 1 by a plug 4 which is preferably provided with perforations topermit the escape of gaseous chemical reaction products. The tube isprovided with a number of equally spaced lateral probes 5, 6, 7, 8 whichpierce its wall and extend through the acid 2 into proximity with thewire 3. At the lower end of the tube 1 there are provided two additionalprobes 9, of the same construction as the probes 5, 6, 7, 8 which extendthrough the acid into close proximity with the lower end of the wire 3.Adjacent to that part of the wire 3 which lies above the uppermost oneof the equally spaced probes 5, 6, '7, 8 is another auxiliary probe 11,likewise of the same construction. Preferably, it is spaced from theprobe 8 less widely than are the probes 5, 6, 7, 8 among themselves. Theauxiliary probe 9 serves to initiate an electrochemical disturbance. Theauxiliary probe 10 serves to preset the apparatus for its codeconversion operations just before the commencement of each code pulsegroup. The auxiliary probe 11 initiates the operation of reading out theconverted code.

Each of the several probes 5, 6, 7, 8 is connected to the anodes of allthe members of a group of rectifiers 12, 13, 14, 15 (Fig. 3) and theserectifiers in turn are connected by way of a matrix to coincidencedetectors CD1 to CD15 of Fig. 3, each of which may include a relay 16 ofFig. 1. The probe 9 is connected by way of a clipper 41 comprising arectifier 17 and a battery 18 and one or more pulse transformers 19, 20and a source of incoming pulses 21 to ground. The upper end of the wire3 is connected directly to ground.

The concentration of the nitric acid 2 is preferably between 50 and 70percent. In other words, its specific gravity lies in the range 1.3 to1.4. When acid of specific gravity below this range is employed, itreadily dissolves the iron wire. When acid of specific gravity 1.3 ormore is employed, a protective film commences to form on the wireimmediately upon immersion of the wire in the acid and very soon reachessuch a thickness that substantially no further corrosion of the irontakes place. When acid of a concentration above the recommended range isemployed, this film is highly stable and the initiation of a disturbanceis difficult. When, however, the concentration of the acid lies withinthe recommended range the film may be readily broken down by applicationof an electrical pulse of a few volts magnitude and enduring for secondor so.

Once the local breakdown has taken place, the acid makes contact withthe iron and a voltaic cell is formed in which the iron wire providesthe anode, the nitric acid the electrolyte and the protective film thecathode. With this arrangement of the materials, depicted in Fig. 2, anelectrical circuit is completed at the interface between the protectivefilm and the iron below it, thus short-circuiting the local voltaiccell. Local galvanic currents then flow primarily in the path XYZ. Inthe vicinity of the point Y the direction of current flow is such as tocause the protective film to be removed by cathodic reduction. At theregion X of Fig. 2 the iron (anode) is again oxidized by a series ofchemical reactions that ultimately generate a new protective film. As aresult of this process the unprotected part of the iron wire advancessteadily in the direction of the protective film, carrying the localvoltaic cell with it. The local voltaic cell currents which thus travelalong the wire constitute an elec- '4 trical disturbance which thusadvances along the wire as a wave.

The initial breakdown is preferably initiated at one end of the wire 3,e. g., at its lower end, as by application of a pulse of electricpotential, whose amplitude exceeds a preassigned threshold determined bythe clipper 4-1, across the film at the lower end of the wire. Thisbreakdown then travels as a propagated wave of sharply definedboundaries and with a minimum of distortion or change of form to theupper free surface of the acid 2 where it disappears. in passing each ofthe probes 10, 5, 6, 7, 8, 11, it gives rise to a pulse of similar waveform and of about 0.7 volt magnitude.

The remaining features and details of the apparatus will be described byreference to Figs. 1 and 3 taken together, of which Fig. 3 shows thedetails of the code conversion matrix by way of which the severallateral pickup probes 5, 6, 7, 8 are connected to the severalcoincidence detectors CD1 to CD15. One construction for such coincidencedetectors is shown, together with the interconnections among itscomponents, in greater detail in Fig. 1 as comprising similarly numberedrelays 16, capacitors 23, and lamps 22.

In the example shown, the apparatus is constructed for conversion fromfour-place binary time code into numerical space code. That is to say,in operation, one and only one of the coincidence detectors CD isactuated for each possible permutation of the pulses of an incoming timecode pulse group. in such a case the transmission line 1, 2, 3 isprovided with four probes S, 6, 7, 3, one for each binary place, andeach of these is connected to all of the anodes of one of four groups ofeight rectifiers 12, 13, 14 and 15, respectively whose cathodes are inturn multipled in the fashion shown in Fig. 3 to the coincidencedetectors CD. Evidently the system could be extended to higher numbers.For example, to convert from numbers in the five-place binary code, fivelateral probes would be provided and each of these would be connected tothe anodes of a group of sixteen rectifiers whose cathodes would in turnbe multipled by way of a matrix of the same general pattern as that ofFig. 3 to 31 load devices.

The operation will be illustrated in connection with the conversion ofthe four-place code pulse group 0011 which in the conventional binarynotation tands for the number three. This code pulse group is shownalongside of Fig. 1 preceded by a marker pulse whose amplitude is two orthree times as great as that of the code pulses and which is preferablyof very short duration. It is therefore illustrated as a sharp narrowspike. Such a sharp narrow spike may easily be caused to precede eachcode pulse group by apparatus which may be located at a transmitterstation and which may, for example, be as indicated in Fig. 4. Here asignal, for example a voice wave originating in a telephone transmitter30, is converted into a code pulse group by a coder 31 whose operationis controlled by a master timing generator 32. The latter may generate awave of sinusoidal form as indicated below the generator 32. This isconverted substantially into a square wave by a slicer 33 whose outputis in turn converted into a sequence of sharp positive and negativespikes by a differentiator 34. A frequency dividing multivibrator 35 isthen provided which responds, for example, to every fifth positivespike, being insensitive to the negative spikes. It therefore deliverspulses of one sign at a rate /5 of that of the timing wave generator 32.Such pulses are differentiated, 36, to sharpen them, the negativeexcursions being then removed by a clipper 37 to leave a sequence ofsharp spikes of one sign. These are inserted in the outgoing pulse trainby a mixer 38 and are precisely located on the time scale with respectthereto by the inclusion of a delay equalizer 39.

Assume that the code pulse group to be converted is of the form 0011 andpreceded by the sharp positivegoing marker pulse as shown in Fig. l. Theclipper 41 is adjusted by selection of the magnitude of the battery 18to pass only voltages in excess of the code pulse amplitudes. Ittherefore passes a pulse each time a marker pulse arrives but blocks thecode pulses. This marker pulse, applied by way of the input probe 9,originates a disturbance which travels in the fashion described abovefrom the lower end of the wire 3 to its upper end giving rise to anoutput pulse on each of the lateral pickup probes in turn. As it passesby the first lateral probe 10, which is auxiliary in the sense that itdoes not form a part of the binary numbering system, it gives rise to anoutput pulse on the probe 10. This pulse energizes the winding of amulticontact relay 24 and the closure of its contacts applies thevoltage of a battery 25 simultaneously to all of a group of condensers23, thus charging them to the battery voltage. This may be termed apresetting operation which prepares the remainder of the system toreceive and recognize an incoming code pulse group.

The output pulse from the first binary probe 5 reaches all of thecoincidence detectors CD8 through CD15 and no others. The output pulsefrom the second probe 6 reaches the coincidence detectors CD4, CD5, CD6,CD7, CD12, CD13, CD14, CD15 and no others. The output A pulse from thethird probe 7 reaches the coincidence detectors CD2, CD3, CD6, CD7,CD10, CD11, CD14, CD15 and no others. The output pulse from the lastprobe 8 reaches the coincidence detectors CD1, CD3, CD5, CD7, CD2, CD11,CD13, CD15 and no others. Referring again to Fig. 1, each of thecoincidence detectors CD may be a two-winding relay 16 of which thecenter point is grounded. By adjustment of the tension of the contactspring and the ampere turns of the winding, the sensitivity may be soadjusted that a pulse through one of these windings sulfices to closethe relay contacts, thus discharging the condenser 23 which is connectedacross these contacts. When, however, pulses of the same polarity areapplied to both the windings of any such relay at the same time, thecontacts remain open and the condenser 23 remains charged.

The incoming code pulse group also passes by way of a transformer 26 anda limiter 27 to the second windings of all of the several relays 16 inparallel. The limiter 27 serves to reduce the amplitude of the markerpulse to that of the code pulses. As explained above, its duration isvery short compared with the duration of any code pulse. When itsamplitude has been thus limited, therefore, its energy is small comparedwith that of any code pulse. The relay windings 16 are proportioned insuch a way that they are sensitive to the energy of a code pulse but notto the lesser energy of the limited marker pulse. With this arrangementit is evident that the second windings of the relays 16 remainunenergized during the first pulse interval and the second but areenergized during the third pulse interval and the fourth. From theinterconnections of the matrix of Fig. 3 it will be apparent that everyrelay 16 of the group is energized at one time or another during thecourse of progress of the electrochemical disturbance from one end ofthe transmission line to the other With the single exception of the oneidentified as 16-3. This one receives energy on its first winding fromthe electrochemical disturbance as it passes the third probe 7. At thesame instant it receives energy from the third code pulse on its secondWinding, and the magnetic pulls of the two windings are balanced so thatthe contacts remain open. Similarly, at the fourth pulse interval thesame relay 16-3 receives energy from the propagated electrochemicaldisturbance as it passes the fourth lateral probe 8 and, by way of thetransformer 26, from the fourth code pulse. The two resulting magneticpulls are again balanced, and the contacts continue to remain open.Inasmuch as the third relay 16-3 is not connected to the first probe 5or to the second 6 at all, and inasmuch, further, as the second windingof this relay receives no energy from the incoming pulse group at thefirst pulse interval or at the second, it remains completely unenergizedthroughout the entire code pulse group, and it is the only one of thegroup which does so. Consequently the condenser 23-3 is the only one ofthe group 23 which remains charged throughout the code pulse group.

As the electrochemical disturbance proceeds beyond the last outputprobe, it passes the read-out probe 11 and a pulse passes by way of arectifier 28 to the winding of the read-out relay 29. The pull of thisrelay closes all of the associated contacts, shown in Fig. l as three innumber but in fact, in four-place binary apparatus, fifteen in number.Each of these contacts interconnects the ungrounded terminal of one ofthe condensers 23 through one of the lamps 22 to ground. In theparticular example under consideration, connection of the thirdcondenser 23-3 to the third lamp 22-3 discharges the condenser throughthe lamp and causes it to fiash. This lamp may be provided in well knownfashion with an opaque mask through which is cut an aperture of the formof the Arabic numeral 3. Closure of the other contacts of the relay 29likewise establishes connections from the ungrounded terminals of theother condensers 23 to their correspondingly numbered lamps 22. However,as explained above, each of these condensers 23, with the sole exceptionof the third one 233, has already been discharged either by theenergization of the first relay winding through the action of thepropagated pulse or by the energization of the second relay windingthrough the action of the code pulse. Hence, one and only one of thelamps 22 of the bank is caused to flash and so makes known its identityand therefore the translated value or meaning of the incoming binarytime code pulse group.

In the foregoing illustrative embodiment, the start pulse whichinitiates the electrochemical disturbance is shown as being delivered atone end of an elongated conductor whereupon it travel as a wave in onlyone direction, namely, to the other end of the elongated conductor.Evidently, however, if the start pulse were to be delivered to theconductor at some point removed from both ends, two disturbances wouldtravel in opposite directions toward the respective ends of theelongated conductor. If desired, each of these disturbances may beindividually turned to account by application of the principles andtechniques described hereinabove.

The foregoing illustrative embodiment of the invention involvestranslation from one code language to another as well as conversion fromtime code to space code, and the examples employed were the binary codeand the decimal code. It will be readily understood, however, that withappropriate changes of the electrical connections between the lateralprobes and the output load devices, any one of a number of other codesmay serve as the code language to be translated or as the translatedcode language. Examples of such other code languages are the reflectedbinary code and the well known two-out-offive code. Since the necessarycircuit changes are entirely straightforward, no further detaileddescription of such translators is called for.

What is claimed is:

1. Apparatus for converting an incoming group of pulses which arearranged sequentially in time to represent a number in accordance with apreassigned permutation code and are preceded by a distinguishablemarker pulse into a unique designation of said number, which comprises apulse propagation line having an input point and a first plurality ofuniformly spaced lateral output taps, a second plurality of conductorsconnected to each of said lateral taps, a third plurality of coincidencedetectors each having first and second input terminals, for indicating,by simultaneous energization of both of said terminals, balance ofsignals applied to said first and second terminals, respectively, amatrix of cross connections interconnecting the first terminals of saiddetectors 7 With said conductors in accordance with a preassigned code,means for applying said marker pulse to said input point as a startpulse, whereupon said start pulse is propagated as a wave along saidline from said input point, giving rise to an output pulse on each ofsaid lateral taps in its passage thereby, means for applying the pulsesof said code group to the second terminals of all of said detectors, andmeans responsive to such balance, in any one of said detectors at everypulse position of said code group, for indicating the identity of saidone detector.

2. vIn combination With apparatus as defined in claim 1, a rectifierconnected in series with each of said eonductors.

3. Apparatus as defined in claim 1 wherein said pulse propagation linecomprises an elongated conductor having a chemically active surface, achemically active medium surrounding said conductor and of aconstitution to react with the material of said conductor to formthereon a protective insulating film, wherein each of said lateral tapscomprises a probe penetrating said medium in proximity with saidconductor and spaced along its length, wherein said input pointcomprises a probe penetrating said medium in proximity to a first end ofsaid elongated conductor, and wherein the application of said startpulse to said input probe effects a local breakdown of that part of saidfilm which protects said first end, whereupon said breakdown travels asan electrochemical disturbance along said elongated conductor and to theother end thereof, giving rise to an output pulse on each of saidlateral probes in its passage thereby.

4. Apparatus as defined in claim 1 wherein said pulse propagation linecomprises a first elongated medium having a preassigncd chemicalconstitution and a surface, a second elongated medium having a chemicalconstitution such as to react with the first medium, disposed in contactwith the first medium in a fashion to provide an interface between saidmedia, said interface having formed thereon, due to chemical reactionbetween said media, a protective insulating film, wherein each of saidlateral taps comprises a probe penetrating said second medium inproximity with said interface and spaced along its length, wherein saidinput point comprises an input probe penetrating said second medium inproximity to a first end of said interface, wherein the application ofsaid start pulse to said input probe effects a local breakdown of thatpart of said film which protects said first end, whereupon saidbreakdown travels as an electrochemical disturbance along said interfaceand to the other thereof, giving rise to an output pulse on each of saidlateral probes in its passage thereby.

5. Apparatus as defined in claim 4 wherein said first medium is ametallic conductor.

6. Apparatus as defined in claim 4 wherein said second medium is anacidic liquid.

7. Apparaus as defined in claim 4 wherein said first and second mediaare coaxially disposed, the second medium surrounding the first medium.

8. Apparatus as defined in claim 1 wherein said pulse propagation linecomprises a first extended medium having a preassigned chemicalconstitution and a surface, a second extended medium having a chemicalconstitution such as to react with the first medium, disposed in contactwith the first medium in a fashion to provide an interface between saidmedia, said interface being characterized by a quasi-stable condition inthe absence of a disturbance applied thereto, wherein each of saidlateral taps comprises a probe penetrating said second medium inproximity with said interface and spaced along its extent, wherein saidinput point comprises an input probe penetrating said second medium inproximity to a part of said interface, and wherein the application ofsaid start pulse to said input probe effects a local disturbance of saidquasi-stable condition, whereupon said disturbance travels as anelectrochemical wave along said interface and to other parts thereof,giving rise to an output pulse on each of said lateral probes in itspassage thereby.

9. Apparatus for converting an incoming group of binary pulses which arearranged sequentially in time and are preceded by a distinguishablemarker pulse into a unique designation of the number represented in thebinary numeration system by said pulses which comprises a pulsepropagation line having an input point and number ll of uniformly spacedlateral output taps, a number m:2"* of conductors connected to each ofsaid lateral taps, a number N:2" of coincidence detectors, 2. matrix ofcross connections interconnecting said detectors with said conductors inaccordance with a preassigned code, means for applying said marker pulseto said input point as a start pulse, whereupon said start pulse ispropagated as a wave along said line from said input point, giving riseto an output pulse on each of said lateral taps in its passage thereby,means for applying the pulses of said code group to all of saiddetectors in opposite phase, and means responsive to balance, at eachpulse position of said code group, of the energies applied to any one ofsaid detectors for indicating the identity of said last-named detector.

10. Apparatus for converting an incoming group of binary pulses whichare arranged sequentially in time to represent a number in accordancewith a preassigned permutation code and are preceded by adistinguishable marker pulse into a unique designation of said number,which comprises a pulse propagation line having an input point and afirst plurality of uniformly spaced lateral output taps, a secondplurality of conductors connected to each of said lateral taps, a thirdplurality of condensers, means for charging all of said condensers, arelay associated with each of said condensers for discharging it, amatrix of cross connections interconnecting said relays with saidconductors in accordance with said preassigned code, means for applyingsaid marker pulse to said input point as a start pulse, whereupon saidstart pulse is propagated as a Wave along said line from said inputpoint, giving rise to an output pulse on each of said lateral taps inits passage thereby and so energizing selected ones of said relays inone phase, means for applying the pulses of said code group to all ofsaid relays in opposite phase, and means responsive to balance, at eachpulse position of said code group, of the energies applied to any one ofsaid relays for indicating the identity of a condenser which remainscharged throughout said code pulse group.

11. Apparatus for converting a time coded pulse group into a space codedsignal which comprises means for applying a distinguishable marker pulsepreceding said time coded pulse group to an input point, a pulsepropagation line connected to said input point, a first plurality oflateral taps spaced along said pulse propagation line, a secondplurality of means, each having two independent input terminals, forpassively indicating, by simultaneous energization of both of saidterminals, chronological balance of signals applied to said terminalsrespectively, connections from one terminal of each of said indicatingmeans to distinctive combinations of said lateral taps and means forapplying said time coded pulse group to the alternate terminal of eachof said indicating means,

whereby said time coded pulse group is converted to a distinctivecombination of those of said individual indicating means which are thussimultaneously energized.

References Cited in the file of this patent UNITED STATE PATENTS2,006,582 Callahan et a1. July 2, 1935 2,403,561 Smith July 9, 19462,616,965 Hoeppner Nov. 4, 1952

